This invention relates to methods and apparatus for electroless plating on substrates. More specifically, the invention relates to improved methods for controlling heat load and airflow during electroless plating in order to better control bath chemistry and plating uniformity.
The Damascene process provides inlaid copper lines in dielectric layers of integrated circuits. The copper lines provide electrical routing (metal interconnects) between circuit elements in the integrated circuit. Damascene copper lines are rapidly replacing traditional aluminum etched lines in high-performance integrated circuitry.
Currently, a preferred method of metal-interconnect layer deposition is electroplating. This is in part due to the success of xe2x80x9cbottom-upxe2x80x9d copper filling methods for damascene features. The process typically involves formation of a barrier layer (typically composed of Ta, TiN or TiSiN) and a seed layer (typically copper) over the wafer, followed by plating the wafer to fill embedded structures from the bottom-up. A number of problems occur when trying to accomplish this task. Such problems include: corrosion of the seed layer and associated reactions in the plating bath, poor structures (morphology) of PVD-deposited seed layers, non-uniform deposition of the metal over and into features, and shrinking of feature volume (and associated increase in aspect ratio) when seeded.
As features become smaller, the seed layer must become thinner (otherwise the feature will be closed off by the generally non-conformal PVD seeding process). However, most useful electroplating tool designs require supplying current to the wafer from the wafer""s outer edge via the seed layer. When attempting to electroplate using ever thinner seed layers for supplying plating current, the current distribution becomes increasingly dominated by the resistance in the seed layer. This phenomenon is commonly referred to as the xe2x80x9cterminal effect.xe2x80x9d
Thus, the need exists to find methods of depositing seed layers in a more conformal manner. This is because conformal seed layers reduce resistance by providing a greater average thickness in comparison non-conformal layers deposited by PVD, for example. As a result, the terminal effect is mitigated during electroplating.
Electroless plating can provide highly conformal seed layers. And in some cases, electroless plating can replace not only PVD seed deposition, but electroplating as well, thereby dispensing with the need for a plating current and a seed layer. This, of course, circumvents the problems of the terminal effect and poor seed layer step coverage.
There are however problems associated with conventional electroless plating methods and apparatus. Electroless plating baths are typically unstable, undergoing varying degrees of homogeneous and heterogeneous plating (depending on such things as bath composition, purity, and temperature). Additionally, decomposition of electroless plating bath reagents can contaminate the plating process. This decomposition is related to the unstable nature of electroless bath reagents when exposed to heat and air, for example.
The problems of bath instability due to heating are well known. For example, Ang (U.S. Pat. Nos. 6,093,451 and 5,938,845) describe an electroless bath heating apparatus designed to uniformly heat an electroless bath for improved plating uniformity. The problem of bath decomposition onto vessel walls was discussed by Cardin et al. (U.S. Pat. No. 4,674,440). They describe a bath design that allows the introduction of a plating bath poison near the bath wall.
Bath degradation and plating uniformity problems are also described in U.S. Pat. No. 6,065,424 by Shaeham-Diamond et al. They describe a spray electroless apparatus and process. In an early patent by the same authors (U.S. Pat. No. 5,830,805) a single sealed chamber is used to perform a number of electroless deposition spray related steps. In both of these patents, mixing the unstable chemicals just prior to use is said to mitigate instability (point of use mixing). However, problems associated with mixing unstable chemicals and spraying them on a wafer are: 1) high capital cost of precision flow mixing and in line (high rate) heating and control, 2) large volume of expensive chemical used in the spraying process, 3) inability of the thermal capacity of the fluid in the spraying process to rapidly or efficiently heat the wafer (which is initially at ambient temperature) to the electroless plating temperature, and 4) metal, formed from the electroless deposition or sprayed solution on the walls of the chamber, produces numerable sources of in film defects. It is necessary to keep the wafer constantly wet during the spinning and/or spraying process. This requires an excess of plating fluid per wafer that is not recovered, and such methods are therefore wasteful. Also, the wafer itself has a substantial thermal heat capacity. Though the wafer surface is being heated by hot spraying chemical, heat is being removed from the surface and being absorbed by the wafer. The process therefore undergoes a temperature transient that is difficult to control, and slows the overall plating rate and thereby reduces throughput.
Along with heat load on an electroless plating formulation, there is decomposition due to air exposure. This problem has two aspects. First, exposure of electroless plating formulations to air can degrade certain components by oxidation. Second, evaporation of bath components cools the bath and requires additional heating, which can accelerate decomposition. Evaporation is a particular problem because most processes will heat their electroless plating fluids to drive plating reactions. Exposure to air cools the electroless plating fluid, due to evaporation, and thus the fluid needs to be heated above desirable temperatures in order to compensate for the evaporative cooling. This additional heating only speeds up the decomposition pathways.
General process requirements for wafer plating include global and local plating uniformity, defect free process, and high throughput. In order realize high-volume manufacturing (e.g. damascene copper processing) meeting these requirements, electroless plating processes must overcome the bath instability issues. In this respect, electroless bath stability can impact both cost of ownership and defect formation. Therefore methods and apparatus that minimize heat and air exposure to the bath are needed. Methods and hardware design that allow improved bath stability, independent of bath composition, are required.
What is therefore needed are improved apparatus and methods for controlling heat load and airflow to which electroless plating fluids are exposed during electroless plating in order to better control bath chemistry, particularly decomposition pathways, and thereby improve plating uniformity and throughput.
Methods and apparatus for reducing the heat load and air exposure to an electroless plating fluid during a plating process are presented. An electroless plating apparatus, including an electroless plating vessel and recirculation systems, is presented. The electroless plating vessel minimizes air exposure (and thus evaporative cooling and degradation) of the electroless plating fluid while the recirculation systems minimize heat load of the electroless plating fluid.
One aspect of the invention is an electroless plating apparatus for reducing loss of electroless plating components from an electroless plating fluid used during an electroless plating process and/or reducing the total heat load imparted to the electroless plating fluid. Such apparatus may be characterized by the following features: a heat exchange recirculation loop, the heat exchange recirculation loop including a heat exchanger and a fluid pump, an inlet and an outlet of the heat exchange recirculation loop configured in fluid communication with: an electroless plating vessel recirculation loop, said electroless plating vessel recirculation loop comprising an electroless plating vessel and configured such that the electroless plating fluid enters the electroless plating vessel recirculation loop via the outlet of the heat exchange recirculation loop and exits the electroless plating vessel recirculation loop via the inlet of the heat exchange recirculation loop; a flow meter configured to measure the flow rate of the electroless plating fluid in the electroless plating vessel recirculation loop; and a flow controller configured to receive flow data input from the flow meter and adjust the amount of the electroless plating fluid that flows through each of the heat exchange recirculation loop and the electroless plating vessel recirculation loop. Preferably the flow controller is configured to maintain the volumetric flow velocity within the heat exchange recirculation loop at a greater value than the volumetric flow velocity within the electroless plating vessel recirculation loop during the electroless plating process. Also preferably, the fluid pump is a fixed-speed centrifugal pump. Heat exchangers of the invention may include one or more heater elements in which the liquid being heated circulates around, and/or travels through the heat transfer element. The heat exchange mechanism may take various forms, such as resistive element heaters, and/or radiating elements like quarts lamps, or a liquid heat exchanger with vapor, liquid or air used as the heat transfer means.
Preferably, the electroless plating vessel includes: a bubble separation chamber, configured to prevent bubbles from entering the plating chamber during the electroless plating process; an airflow control device, configured to modulate exposure of the electroless plating fluid to air during the electroless plating process; and a ballast tank, configured to receive the electroless plating fluid from a plating chamber of the electroless plating vessel and return the electroless plating fluid to the heat exchange recirculation loop. Preferably the electroless plating vessel further includes a fluid flow diffuser configured to create a uniform laminar flow of the electroless plating fluid which impinging upon a substantially planar work surface of a workpiece during the electroless plating process.
Also preferably the electroless plating cell further includes a rinse return duct, configured to receive rinsate from the workpiece during rinse operations. The rinse return duct may also serve as an airflow exhaust port. In a preferred embodiment, the rinse return duct feeds rinsate and exhaust air into an air and rinsate drain manifold configured to feed air and rinsate into separate waste streams. In another preferred embodiment, the rinse return duct further includes an airflow exhaust control distributor configured to maintain a substantially even flow distribution in the total airflow within the electroless plating vessel during the electroless plating process.
Preferably the airflow control device is an airflow confinement ring attached to a wafer holder used to plate wafers in the electroless plating vessel. The airflow confinement ring, when positioned within the plating cell, creates an airflow channel (in conjunction with the inner walls of the plating vessel for example) through which airflow passes. Preferably the wafer holder is a clamshell wafer holder. Preferably the airflow confinement ring is configured to minimize the amount of air to pass over the electroless plating fluid during any stage of the electroless plating process, while still containing any vapors that may be produced by the liquid.
Preferably the ballast tank includes a fluid return slide configured to allow the electroless plating fluid to enter the ballast tank along a slope and thereby minimize agitation of the electroless plating fluid during return to the ballast tank.
Another aspect of the invention is the electroless plating vessel as described above, and below in relation to the figures which show an exemplary electroless plating vessel.
Yet another aspect of the invention is a method of reducing the total head load of an electroless plating fluid during a plating process. Such methods may be characterized by the following aspects: circulating the electroless plating fluid through a heat exchange recirculation loop at a first volumetric flow velocity, the heat exchange recirculation loop including a heat exchanger and a fluid pump, an inlet and an outlet of the heat exchange recirculation loop configured in fluid communication with an electroless plating vessel recirculation loop; and circulating the electroless plating fluid through the electroless plating vessel recirculation loop at a second volumetric flow velocity, the electroless plating vessel recirculation loop including an electroless plating vessel and configured such that the electroless plating fluid enters the electroless plating vessel recirculation loop via the outlet of the heat exchange recirculation loop and exits the electroless plating vessel recirculation loop via the inlet of the heat exchange recirculation loop. In such methods, preferably the first volumetric flow velocity is greater than the second volumetric flow velocity. Preferably the first volumetric flow velocity is between about 2 lpm and 12 lpm, and the second volumetric flow velocity is between about 0.5 lpm and 6 lpm. Preferably such methods further include controlling the amount of evaporation of the electroless plating fluid during plating. One preferred method of controlling evaporation is by minimizing the airflow to which the electroless plating fluid in the electroless plating vessel is exposed. One preferred method of minimizing the airflow to which the electroless plating fluid is exposed includes use of an airflow confinement ring attached to a wafer holder and configured to provide, in conjunction with an inner surface of the electroless plating vessel, a peripheral air channel through which air flows during the plating process.
Yet another aspect of the invention is a method of removing metal deposits from surfaces of a wafer plating apparatus. Such methods are particularly useful for electroless plating applications, especially those carried out in apparatus such as described above and in relation to the figures below. Such methods may be characterized by the following aspects: removing a plating solution from the wafer plating apparatus; providing an inorganic acidic medium to the wafer plating apparatus; providing an oxidizing medium to the wafer plating apparatus; circulating the inorganic acidic medium and the oxidizing medium through the wafer plating apparatus; and removing the inorganic acidic medium and the oxidizing medium from the wafer plating apparatus.
Preferably the acidic medium includes at least one of sulfuric acid, hydrochloric acid, nitric acid, phosphomolybdic acid, perchloric acid, and mixtures thereof. Preferably the oxidizing medium includes at least one of hydrogen peroxide, potassium permanganate, chromate salts, ozone, perchlorate salts, and mixtures thereof. Other acids and oxidizing agent can also be used as known by those skilled in the art. In a particularly preferred embodiment, the inorganic acid includes between about 1 and 5 weight percent sulfuric acid, and the oxidizing medium includes between about 1 and 5 weight percent of aqueous hydrogen peroxide. Also preferably such methods further include rinsing the wafer plating apparatus with deionized water after removing the inorganic acidic medium and the oxidizing medium from the wafer plating apparatus.